Larger intake/exhaust ports, richer jetting in the carburetors, plus "power-ported" pistons and block. Allowing the 115 to flow a lot more air and make more HP.
The mid-80's 115 is prop-rated and if you were to find an equivalent crankshaft-rated engine, it'd be an older (circa 1979 or so) 140hp Six. The mid-80's 90hp would be the equivalent of an earlier crank-rated 115hp.
The late-80's 115's were the best of the Inlines and probably pumped out more than 140hp at the crank, likely closer to an older 1500. Real strong runners.
If you're thinking you might upgrade a 90hp to a 115, you're better off finding a 115 if you need that extra power.
HTH...........ed
